I have seen a sticker about the place, that says , "Gaeltacht bheag an carr seo".

Can someone explain to me why you'd use "an" rather than "sa", is it saying the car itself is a Gaeltacht ?

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Bíonn rudaí maithe mall

Yes. The copula "is" is understood at the beginning of the sentence.

And is ordinarily ok to omit it at the beginning of a phrase or sentence?

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Bíonn rudaí maithe mall

Depends on the context. It's generally ok to omit it in the present tense in "classification sentences" such as: 'Bean álainn (í) Máire'; 'Múinteoir scoile (é) mo dheartháir'.

In fronted sentences: 'Ag magadh a bhí mé'.

Often in proverbs: 'Tús maith, leath na hoibre'.

And other contexts too.

A classic instance of the copula being dropped at the beginning of a sentence is in introductions. For example, you don't normally say "Is mise Dáithí"...you'd just say "Mise Dáithí." If you were introducing a friend named "Máire," you'd just say "Seo Máire."

Redwolf

Yeah I think that's what they are saying. The car, i.e. the location, is the Gaeltacht. And the people are in it.

Seems a bit odd to me , the car being the Gaeltacht, the omission of the copula I get. Although l would have thought you include it in writing, but as I do in English drop parts that are implied in conversation.

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Bíonn rudaí maithe mall

It's all in fun. The idea is that the people in the car (or at least the person who owns the car) speaks Irish and enjoys driving around in it with other Irish speakers. I used to have one on my old car (next one that said "Bí cúramach! Tiománaí ina chodladh")

Redwolf